1. Field of the Invention
The present invention relates to concrete reinforcing steel fibers to be mixed in concrete for reinforcing concrete and to a method of manufacturing such concrete reinforcing steel fibers.
2. Description of the Prior Art
In recent years, various composite materials having new properties which have never been found in individual materials have been developed. In the civil engineering and architectural industries, it has been desired to improve the properties of concrete which is inexpensive and has both many advantages and many disadvantages such as brittleness and liability to fissure. Recently, steel fiber reinforced concrete (hereinafter referred to simply as "reinforced concrete") having high tensile strength and very high toughness has been developed and become fully applied to practical uses. Generally, such a reinforced concrete contains 1 to 2% by volume (80 to 160 kg/m.sup.3) steel fibers having a diameter in the range of 0.4 to 0.6 mm and a length in the ranage of 20 to 60 mm. Various researches and development activities have been conducted in the fields of steel fiber manufacturing techniques, steel fiber application techniques and the utilization of steel fibers and the reinforced concrete.
Incidentally, in order to prepare the reinforced concrete containing steel fibers in the above-mentioned volume ratio, 80 to 160 kg of steel fibers per one cubic meter of concrete are necessary, and hence in placing the reinforced concrete in the practical construction work, a large quantity of steel fibers must be supplied.
The following steel fiber manufacturing methods have been proposed for the mass production of steel fibers.
(1) A thin plate shearing method in which a coil of cold-rolled thin steel plate having a thickness in the range of 0.2 to 0.7 mm is cut with a rotary blade.
(2) A thin plate cutting method in which hundreds of thin steel sheets having a thickness in the range of 0.2 to 0.7 mm are placed one over another in a pile, and then the pile of thin steel sheets is cut with a cutter on a planer.
(3) A wire cutting method in which drawn steel wires having an appropriate diameter are cut in a predetermined length.
(4) A cutting method in which steel slabs or steel ingots are cut on a planer.
(5) A centrifugal method in which a water-cooled rotary disk provided with threads in the circmference thereof is placed in contact with the ssurface of molten steel to draw out the molten steel for instantaneous solidification in steel fibers and to scatter the steel fibers by centrifugal force.
The steel fibers manufactured by those steel fiber manufacturing methods, respectively, are different from each other in characteristics, and provides the reinforced concrete with different strength characteristics, respectively.
The adhesion of the steel fibers to concrete is considered to be a direct and dominant factor affecting the strength of the reinforced concrete. Although the tensile strength of the steel fibers can be easily measured, it is difficult to measure the strength of adhesion of the steel fibers to concrete in the reinforced concrete.
The function of steel fibers in an ordinary reinforced concrete not prestressed is to adhere to concrete, to take charge of a portion of the load acting on the reinforced concrete, to reinforce the concrete so that cracks will not develop, and to reinforce the concrete so that the concrete will not break.
Basically, steel fibers are used for the above-mentioned function, however, since steel fibers are more capable of being integrated with concrete than steel bars, steel fibers are expected to absorb as much energy as possible when the reinforced concrete is broken and to provide the reinforced concrete structure with high toughness.
To enable steel fibers exhibit the excellent performance and to produce steel fibers of high performance at a reduced cost, the steel fibers must be well balanced in characteristics, namely, the tensile strength must be equal to the adhesion to concrete. Such a condition is explained theoretically by EQU .pi..multidot.d.multidot.l/4.multidot..tau.=.pi.d.sup.2 /4.multidot..sigma..sub.f, hence, .sigma..sub.f =.tau.d
where d=the diameter of the steel fiber, l=the length of the steel fiber, .tau.=the strength of adhesion of the steel fiber to concrete, and .sigma..sub.f =the tensile strength of the steel fiber.
If the strength of adhesion of the steel fiber to concrete is excessively large, EQU .pi..multidot.d.multidot.l/4.multidot..tau.&gt;.pi.d.sup.2 /4.multidot..sigma..sub.f
Therefore, when the reinforced concrete is loaded, the steel fibers will be broken before being extracted from the reinforced concrete.
If the strength of adhesion of the steel fiber to concrete is excessively small, EQU .pi..multidot.d.multidot.l/4.multidot..tau.&lt;.pi.d.sup.2 /4.multidot..sigma..sub.f
Therefore, the steel fibers will be extracted from the reinforced concrete before the steel fibers are broken, and hence the steel fibers are unable to fully exhibit their tensile strength.
In order to enhance the adhesion of the steel fiber to concrete, various designs on the shape of the steel fiber have been proposed. Typical steel fibers having a special shape are:
(a) a steel fiber having a straight axis and varying in cross section along the axis,
(b) a steel fiber having a wavy axis, and
(c) a steel fiber having ends formed in a special shape.
These shapes are experiential shapes and not theoretical shapes. The steel fiber of (b) is unsatisfactory in respect of effecitve length and is uneconomical, while the steel fiber of (c) has difficulty in manufacture. The steel fiber (a) is economical, easy to manufacture and stable and uniform in quality. In manufacturing the steel fiber of (a), it is essential to design the cross section of the section-shaped portions so that the steel fiber is balanced between the tensile strength and the strength of adhesion to concrete. Although the higher the surface irregularlity, the greater the strength of adhesion to concrete, the tensile strength diminishes with increase in the variation of the cross section along the axis. Therefore, the degree of the surface irregularity of the steel fiber must be chosen so that the steel fiber is balanced between the tensile strength and the strength of adhesion to concrete. However, as regards the morphology of the section-shaped portion of the steel fiber, nothing is elucidated as to the preferable degree of expansion of the section shaped portion and measures to enhance the toughness of the reinforced concrete incorporating the steel fibers. Accordingly, steel fibers having a theoretical shape appropriate to the enhancement of the toughness of the reinforced concrete are not produced at the present.